My take is that you failed to understand how chronic homelessness works. Excluding the mentally ill, most of the said "able-bodied" individuals (including drug addicts) are there by choice. They managed to limit their needs and wants to a minimal levels and cover those with almost no work at all. As far as I could understand the way homeless people think, there are only small windows of opportunity for some external motivation to reach them, and that's in the times when they can't cover their basic needs, like shelter in a really cold weather, or water in a really dry day. Even these are looked at as nice-to-have not a must-have, so the effect of leverage is weak, as many of them will rather try to suffer and endure it through at the first sign of required work from their part.

"We claim to "rehabilitate" criminals, so I fail to see why we can't do something on a similar effort-level for the really needy individuals in our society."

I think you again fail at understanding what's the deal with criminals and homeless and why the society works to rehabilitate the criminals but not the homeless. (Actually the homeless that can be rehabilitated are actually worked on, as in the homeless that themselves work on their condition are helped.) The imprisoned criminals are a group of people that for some reason were considered dangerous to be left active in the society. So not necessarily the punishment for their crimes but the threat they continuously pose is the primary reason for their temporary or permanent disposal from the rest of society. Rehabilitation comes as a natural step for these active people, as they are active and potentially still useful for society. The homeless, on the other hand, don't pose much threat to society, and the main problem with any assumed social program involving them is that most of these are not active. There isn't much drive to change anything, neither from society (unless there is something political about it), nor from the homeless themselves.